It is desirable that the indicia overlying flexible touch switches not fade or be entirely rubbed off from frequent operation of the switches or adversely affected by exposure to the environment. It is also desirable to simplify construction.
Leposavic U.S. Pat. No. 3,600,528 and Larson U.S. Pat. No. 4,017,697 describe touch switches having very few layers, but in both the indicia are applied to the top of the overlay. Other touch switches which have relatively few layers but in which indicia are apparently applied to the switch top surface are shown in Lynn et al. U.S. Pat. No. 3,860,771 and Seeger, Jr. et al. U.S. Pat. No. 3,789,167.
Some known touch switches have indicia below the switch top surface. Schmitz et al. U.S. Pat. No. 3,627,927 teaches etching symbol designations in a conductive sheet or conductive pads adhered to the underside of a flexible insulating layer; if pads are used, etching should be done near the perimeter of each of the pads so as not to interfere with the contact area at the center of each of the pads. Bach U.S. Pat. No. 3,811,025; Lynn et al. U.S. Pat. No. 3,978,297; Driver U.S. Pat. No. 3,590,195; and Harris U.S. Pat. No. 3,777,222 all describe touch switches in which indicia are formed on the bottom or intermediate surface of a transparent overlay; however, in all these patents the indicia-carrying overlay is separate from the depressible conductive layer, as is true of Zurcher U.S. Pat. No. 4,028,509 in which metallized indicia are placed on the underside of the stylus-receiving layer for protection from the stylus.
Larson U.S. Pat. No. 3,987,259 teaches applying conductive paint to the underside of a membrane overlay.